A heat exchanger known in the related art adopts a four-pass structure that includes a plurality of tubes disposed in two rows, i.e., a front row and a rear row along the direction of airflow, through which a coolant flows along an up/down direction, an upper tank unit and a lower tank unit respectively communicating with the upper ends and the lower ends of the tubes and the like (see patent reference literature 1). (Patent reference literature 1) Japanese Unexamined Patent Publication No. 2001-74388
In the heat exchanger adopting the four-pass structure described above, the liquid coolant flowing through an upper tank unit 100 tends to be distributed in greater quantity to the tubes located on the upstream side along the coolant distribution direction due to gravity, whereas the coolant flowing through lower tank units 101 and 102 tends to be distributed in greater quantity to the tubes located on the downstream side along the coolant distribution direction due to inertia, as shown in FIG. 8a. This means that the temperatures over an area in a first pass portion 110 on the side toward a second pass portion, an area B at the second pass portion 111, an area C at a third pass portion 112 and an area D at a fourth pass portion 113 rise readily, since the liquid coolant flow rate over these areas is bound to be low. In particular, the temperature of the output air will rise markedly over an area E (see FIG. 8b) over which the first pass portion 110 and the fourth pass portion 113 overlap fore and aft along the direction of airflow. This tendency becomes more pronounced when the coolant flows at a low flow rate. Test results indicate that the temperatures in some of the areas rise as high as 10 to 20° C., adversely affecting temperature control in the cabin.
The problem described above is addressed in the evaporator disclosed in patent reference literature 1 by forming a plurality of restricting holes 51a to 56a at the lower tank units over the second pass portion and the fourth pass portion so as to adjust the coolant flow rate (see patent reference literature 1). However, the cost of the heat exchanger disclosed in patent reference literature 1 adopting a complicated structure in the tanks is bound to increase significantly.
In addition, an inflow port 9 formed at an evaporator-side intake connector is constricted and the inflow port 9 is set on the upper side along the height of the tanks so as to specifically improve the distribution of the coolant flowing at a low flow rate in the heat exchanger disclosed in patent reference literature 2. However, this structural feature cannot be utilized to full advantage at a very low flow rate, e.g., at the full destroke setting in an air-conditioning system that employs a variable-displacement compressor. Furthermore, at the full capacity setting (maximum flow rate), another problem occurs in that a great deal of resistance is created at the constriction. (Patent reference literature 2) Japanese Unexamined Patent Publication No. 2005-156095